1. Field of the Invention
The present invention relates to recording media and, in particular, to a new magneto-optical disk and the method of playback thereof.
2. Description of the Related Art
Recently, great efforts have been made to develop an optical recording and playback method, as well as recording equipment, playback equipment and recording media to be used according to such a method, wherein a variety of demands will be satisfied, including high density and large volume storage, high access speeds, and high recording and playback speeds.
Among the wide range of optical recording and playback methods presently contemplated, the magneto-optical recording and playback method holds the greatest promise because of the unique advantage that, after being recorded on the recording medium, information can be erased, and it is possible to repeatedly record new information on the recording medium many times over.
The magneto-optical recording disk (medium) used in this magneto-optical recording and playback method has a magnetic film comprising one or multiple layers to be the layer that preserves the recording. Originally, when magneto-optical disks were first produced, the magnetic film was a horizontally magnetized film (the direction of magnetization ran parallel to the film plane). Later, perpendicularly magnetized layers, which have high recording densities and high signal strength, were developed. Today, this latter type of magneto-optical recording disk, having perpendicularly magnetized layers, is used most of the time and the magnetized film is composed of, for example, amorphous GdFe and GdCo, GdFeCo, TbFe, TbCo, or TbFeCo, etc. In general, the magneto-optical recording disk has concentric circular or linear tracks, and information is recorded on these tracks.
For a description of the related art herein, a first direction of magnetization (the "upward" direction) in relation to the film plane shall be defined as the "A direction," and a second direction of magnetization, opposite to the first direction of magnetization, shall be defined as the "anti-A direction." The information to be recorded on the magneto-optical disk has previously been made into binary information, and is recorded by two signals, the first being a mark which is magnetized in the "A direction" and designated B.sub.1, and the second being a mark that has been magnetized in the "anti-A direction" and designated B.sub.0. These two marks B.sub.1 and B.sub.0 are equivalent to the digital signals of 1 for one direction and 0 for the other, respectively. However, the magnetizing of the recording track is generally uniform in the "anti-A direction" by applying a powerful external magnetic field prior to recording. This act of making a uniform direction of magnetization is called initialization. In addition, mark B.sub.1, which is magnetized in the "A direction," is formed in a track. One of the marks, B.sub.0 or B.sub.1, is taken as the informational unit, and information is expressed using the presence/absence and/or the length of the informational unit, which is normally mark B.sub.1. In the past, the term "mark" as noted above was called a pit, or a bit, but currently it is being called a mark.
In forming the marks, advantage is taken of the special characteristics of the laser, specifically, the spatially and temporally excellent coherence. The beam of the laser is focused on a small spot in about the same order as the diffractive limit that is determined by the wavelength of the laser light. The focused light irradiates the track surface, and information is recorded by forming a mark with a diameter of 1 .mu.m or less on the perpendicularly magnetized film. Theoretically, a recording density of approximately 10.sup.8 marks/cm.sup.2 can be achieved during optical recording. This is because the laser beam can be concentrated on spots that have a small diameter of about the same order as its wavelength.
As shown in FIG. 2, during magneto-optical recording the disk moves in the direction "C" and, laser beam L is focused on and selectively heats perpendicularly magnetized film M.phi.. In that interval, recording magnetic field Hb, in the direction opposite to the initialized direction, is applied from outside on the heated portion. When this is done, the coercivity Hc of the portion that is locally heated is reduced and becomes smaller than the recording magnetic field Hb. As a result, the magnetization of this portion is lined up in the direction of recording magnetic field Hb. Marks magnetized in the opposite direction are also formed in this way.
FIG. 3 shows the principles of information playback based on magneto-optical effects. Light is an electromagnetic wave which has electromagnetic field vectors that are normally dispersed in all directions on a perpendicular plane in the light path. When light is converted to linear polarized light Lp and is then irradiated on a perpendicularly magnetized film M.phi., the light will either be reflected by that surface, or will pass through the perpendicularly magnetized film. At this time, the plane of polarization will rotate according to the direction of magnetization. The phenomenon of this rotation is called the "magnetic Kerr effect" or the "magnetic Faraday effect."
For example, if the plane of polarization of the reflected light rotates +.theta..sub.K degrees in relation to the "A direction" ("upward direction") magnetization, then it will rotate -.theta..sub.K degrees in relation to the "anti-A direction" ("downward direction") magnetization. Consequently, when the axis of a photo-analyzer (polarizer), not shown in the drawings, is set perpendicularly to the plane tilted -.theta..sub.K degrees, the light that is reflected from mark B.sub.O, which is magnetized in the "anti-A direction," cannot pass through the analyzer. As opposed to this, the portion of the light that is reflected from mark B.sub.1, which is polarized in the "A direction," and that is multiplied by (sin 2.theta..sub.K).sup.2 passes through the analyzer, and is captured by a detector (opto-electric conversion means), also not shown in the drawings. As a result, mark B.sub.1 magnetized in the "A direction" is seen as brighter than mark B.sub.0 which is polarized in the "anti-A direction," and produces a strong electric signal in the detector. Consequently, because the electric signals from this detector are modulated according to the recorded information, the information can be played back.
The perpendicularly magnetized film is formed of magnetic layers which are preferably a non-crystalline ferromagnetic body composed of an alloy of a transitional metal (abbreviated "TM" hereinafter) and a heavy rare earth metal (abbreviated "RE" hereinafter). Examples of TM include Fe and Co, and examples of RE include Gd, Tb, Dy and Ho. The direction and size of the magnetism outside of the alloy is determined by the relationship between the direction and size of the magnetism within the alloy of the sub-lattice magnetization of the TM and the direction and size of the sub-lattice magnetization of the RE as shown in FIG. 4. For example, as shown in FIGS. 5A-5D, the direction and size of the sub-lattice magnetization of TM is expressed by the vector indicated by the dotted line arrow, the sub-lattice magnetization of RE is expressed by the vector indicated by the solid line arrow, and the direction and size of the magnetization of the alloy as a whole is expressed by the vector indicated by the outlined arrow. The outlined arrow (vector) at this time is expressed as the sum of the dotted line arrow (vector) and the solid line arrow (vector). However, because of the mutual action of the sub-lattice magnetization of TM and the sub-lattice magnetization of RE within the alloy, the dotted arrow (vector) and the solid line arrow (vector) always run in opposite directions. Consequently, the sum of the dotted line arrow (vector) and the solid line arrow (vector) is the vector of the alloy, and is zero when both have equal strength (specifically, the size of the magnetism expressed externally is zero). The alloy composition when this is zero is called a "compensation composition." When the composition is other than this, the alloy has the outlined arrow (vector) which has a strength equal to the difference in strength of the sub-lattice magnetizations and has a direction equal to the direction of whichever vector is larger.
Thus, the magnetized vector of the alloy can be expressed by writing the dotted line vector and the solid line vector next to each other as, for example, in FIG. 4. The states of RE and TM sub-lattice magnetization can be broadly classified four ways as is indicated in FIGS. 5A-5D, respectively. The magnetic vectors (outlined arrow) of the alloy for each of these states is shown corresponding to FIGS. 5A-5D. For example, if the RE vector is larger than the TM vector and directed upwards, the states of the sub-lattice magnetizations are indicated by the solid and dotted lines in FIG. 5A, and the magnetic vector of the alloy is indicated by the outlined arrow.
A given alloy composition is called by the name of whichever vector is stronger of the TM vector and the RE vector, and is referred to as TM rich or RE rich. The direction of the magnetization of the alloy as a whole agrees with the direction of the TM (or RE) sub-lattice magnetization of the TM (or RE) rich alloy.
Each of the primary and secondary magnetic layers is divided into a TM rich composition or an RE rich composition. Consequently, when the composition of the primary magnetic layer is taken as the vertical axis coordinate and the composition of the secondary magnetic layer is taken as the horizontal axis coordinate, the disk as a whole can be classified into the four kinds of quadrants indicated in FIG. 6. In FIG. 6, the intersection of the coordinates expresses the compensation composition of both layers.
When the magnetism of the magnetic layers is exchanged coupled, the exchange coupling force .sigma..sub.W works between the magnetic layers. Exchange coupling force .sigma..sub.W acts in the same direction as the direction of the sub-lattice magnetization of the TM and RE, respectively. For this reason, when the magnetism of both layers is exchange coupled there are two kinds of stable two-layer film: (1) a stable two-layer film when the direction of the primary magnetic layer magnetization and the direction of the secondary magnetic layer magnetization are parallel, and (2) a stable two-layer film when the directions of magnetization of the two magnetic layers are anti-parallel. In either case, in a stable state, the directions of the TM sub-lattice magnetization of both magnetic layers agree. Naturally, the directions of RE sub-lattice magnetization of both magnetic layers agree. The first kind of two layer film shall be called the parallel ("P") type and the second kind of two layer film shall be called the anti-parallel ("A") type. The parallel type is unstable when the directions of magnetization of the two layers are anti-parallel, and at that time a magnetic wall is formed between the layers. The anti-parallel type is unstable when the directions of magnetization of the layers are parallel, and at that time a magnetic wall is formed between layers. In the unstable state, the directions of the TM sub-lattice magnetization of both layers do not agree, and the directions of the RE sub-lattice magnetization of both layers do not agree. However, even if it is called "unstable," it is provisionally stable, and that state can persist for a couple of years to several dozen years. The P type belongs to the first and third quadrants indicated in FIG. 6, and the A type belongs to the second and fourth quadrants.
In either case, the magneto-optical recording disks, up to this point in time, have the advantage that it is possible to repeatedly record and playback. However, when the disks are accidentally exposed to an excessive magnetic field or are exposed to an abnormally high temperature, there is the problem that the information may be erased.